This invention relates to EMI shielding pigments. Such pigments have a shielding effect against electric and magnetic fields in organic and inorganic coatings or films and thereby prevent electromagnetic interference (EMI). Coatings containing EMI shielding pigments on the housing or covering of an apparatus, protect an underlying substrate, for example, an electronic part or component, against electromagnetic energy from other sources. These coatings further prevent the escape of electromagnetic energy from the apparatus inside the housing.
It is known that pigments having ferro- or ferrimagnetic properties may be used for shielding against magnetic and low frequency electromagnetic fields. See, for example, German Patent Application No. 37 09 217 and Japanese Patent Application No. 61-078873 A2, which relates to Chemical Abstract No. 105 (16):135126c. These documents are entirely incorporated herein by reference. Non-ferromagnetic pigments which have good electrical conductivity can only be used for shielding against electric fields, and possibly high frequency electromagnetic fields. See, for example, German Patent Application No. 30 28 114 which is also entirely incorporated herein by reference.
For highly effective EMI protection over a wide frequency range of electromagnetic fields, both ferro/ferrimagnetic pigments and electrically conductive pigments are incorporated in the coatings. An example of such coating is a ferrophosphorus alloy and nickel or silver, as disclosed in U.S. Pat. Nos. 4,517,118 and 4,518,524, which patents are entirely incorporated herein by reference. The disadvantages of the pigments in accordance with these patents are the great safety precautions required when working with carcinogenic nickel powder and/or the high cost of the required silver component. Instead of using two different pigments in coating compounds, it is sometimes possible to preserve the conductivity of the ferromagnetic materials and thereby improve the EMI protective action by metallizing a ferro- or ferrimagnetic pigment with a highly conductive metal. Thus, shielding pigments based on spherical or platelet shaped nickel particles which have a first coating of copper and a second coating of silver are disclosed in EP-A 0 343 836 (which is entirely incorporated herein by reference).
Japanese Patent Application No. 63-013303 A2 (which relates to Chemical Abstract No. 109 (14):120978r, each of which are incorporated entirely by reference) discloses highly conductive pigments based on silver-coated magnetite powder. These pigments contain from 30 to 70% by weight of Ag or Ag-Pd alloys, based on the iron oxide put into the process, and are used for the production of printed circuits. The use of these materials as shielding pigments and a platelet shaped structure of the pigments are not disclosed in these documents.
There have been many attempts to obtain shielding pigments having a platelet shaped structure, but known pigments of this type do not fulfil all expectations. Thus, for example, micas from the series of muscovite, phlogopite, sericite and oolithic mica have been metallized current-free. Ni-coated phlogopite, according to Japanese Patent Application No. 62-028497 A2 (which relates to Chemical Abstract No. 107 (10):79832g, which documents are entirely incorporated herein by reference), has a good shielding effect but is toxicologically suspect, like other Ni powders. Cu-coated muscovite and Cu-coated sericite require the use of organosilanes or organotitanates to avoid loss of activity by oxide formation, and the magnetic shielding effect is limited. See, Japanese Patent Application No. 59-086638 A2 (which relates to Chemical Abstract No. 101 (16):132088y) and Japanese Patent Application No. 01-095169 A2 (which relates to Chemical Abstract No. 111 (14):116368r), all of which are entirely incorporated herein by reference. Oolithic mica has also been silvered current-free for the preparation of electrically conductive pigments having a contact resistance below 10.sup.-3 .OMEGA..multidot.cm, as disclosed in Japanese Patent Application No. 63-020486 (which relates to Chemical Abstract No. 108 (22):1912752, each of which is entirely incorporated herein by reference).